Germ layer specification in a diploblast

As one of the earliest cell fate decisions in the development of most animals 3 major germ layers (ectoderm, endoderm and mesoderm) have evolved, which play crucial functions during the embryonic development. Ectoderm will form the outer cell layer of the embryo, while endoderm and mesoderm will be internalized to generate inside cell layers. For the most part, the three germ layers give rise to distinct tissues and differentiated cell types . In this context, the most fundamental tasks are to understand (A) how the transition of cell states and cell types during germ layer formation is regulated and (B) what are the consequences of changes in the regulation of cell states on animal body plans and their evolution. However, cnidarians (sea anemones, corals, jellyfish), which arose early in animal evolution, are diploblastic, i.e. they consist only of two germ layers, commonly termed ectoderm and endoderm. Hence, they seem to lack the mesoderm, despite sharing many genes that have a crucial role in mesoderm formation in animals with three germ layers. Recent work has, however, suggested that the diploblastic cnidarians might have already a topological segregation of endodermal and mesodermal tissue identities. This project seeks to reveal how these tissue identities are separated from ectoderm in cnidarians in comparison to vertebrates and insects, with the final goal to get insights into the evolution of the three germ layers in vertebrates. To this end, we will compare cnidarians and vertebrates by studying key regulatory proteins that drive the activation of endodermal and mesodermal developmental programs. These studies will shed light into key changes in the regulatory network of the developmental genes during animal evolution allowing us to draw conclusions on how this major transition from rather simple cnidarians to more complex animals occurred.

All organs, tissues, and cell types of the human body originate from three germ layers, which are established very early in embryonic development: ectoderm, endoderm, and mesoderm. This also applies to almost all other animals, which are grouped into the so called "Bilateria". However, cnidarians (corals, sea anemones, jellyfish), which diverged from the Bilateria 650 million years ago, consist of only two cell layers, commonly referred to as ectoderm and endoderm. The mesoderm, therefore, appears to have evolved only at the Bilateria stage. To clarify the evolutionary origin of the three germ layers, genes that play an important role in the specification of the three germ layers in Bilateria were examined in sea anemones. Surprisingly, we were able to show that in the two-layered sea anemone, there is already a clear segregation of the three germ layer identities at the molecular level, which apparently only needed to be followed by spatial separation in the divergence of the bilaterians. Many of the genes studied actually have the same molecular function in cnidarians and bilaterians (including humans). Even the interplay of the same signaling pathways implicated in the molecular segregation of the later germ layer domains during early embryogenesis predated the divergence of Cnidaria and Bilateria, some 650 million years.